University of Groningen The role of male courtship …Aitana Peire, Rense Veenstra, Christof Pietsch*, Louis van de Zande, Juergen Gadau**, Leo W. Beukeboom * Institute of Plant Genetics

University of Groningen

The role of male courtship behaviour in prezygotic isolation in NasoniaPeire Morais, Aitana

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6. QTL analysis of Nasonia

male behaviour using interspecific hybrids

Aitana Peire, Rense Veenstra, Christof Pietsch*, Louis van de Zande,Juergen Gadau**, Leo W. Beukeboom

* Institute of Plant Genetics and Crop Plant Research (IPK),Correnstrasse 3, D-06466 Gatersleben, Germany

** School of Life Science, Arizona State University, P.O. Box 874501, AZ 85287-4501 Tempe, USA

06-QTL analysis of Nasonia 19/2/07 11:07 Página 107


Abstract

Phenotypic differences between recently diverged species arecandidate traits causing or enhancing speciation. Unravelling thegenetic architecture of divergent traits might help understanding theevolutionary forces responsible for divergence of ancestral genes.Males of the three sibling Nasonia species, N. vitripennis, N.longicornis and N. giraulti, show variations to a common pattern ofcourtship behaviour, which role in the speciation process needs to beelucidated. We have analysed the phenotype, underlying QTL andepistatic interactions of several components of behaviour in hybridmales originating from the cross between N. vitripennis males and N.longicornis females. The phenotype of hybrid males was biasedtowards N. longicornis in most of the courtship components. QTL forall analysed courtship components were distributed over all fivechromosomes explaining 29-95% of the phenotypic variance. Wefound epistatic interactions between loci responsible for transgressivephenotypes in hybrids in three of the courtship components. A regionof postzygotic incompatibility, expressed as transgressive phenotypesin the hybrids was found on chromosome 1.

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Introduction

Differences in courtship behaviour between closely related speciesmight play a role in the speciation process or might be a product ofreproductive isolation and lack of gene flow. Determining the geneticbasis of behavioural differences helps understanding their evolutionaryrole (Orr 2001; Boake et al. 2002; Merilä and Sheldon 1999). The basicgenetic architecture of a trait, i.e. number and location of loci in thegenome and interaction between loci, can be unravelled withquantitative genetics and quantitative trait loci (QTL) analysis.

Major changes in behaviour might be the consequence of strongselection on a few loci with major effects or the reflection of selectionin phenotypes controlled by many genes with small effect. Differencesdue to small changes in many loci might be the outcome of gradualselection or random drift (Orr 2001). The same applies for non-additiveinteraction between loci (epistasis): while within species epistasis isexpected to modulate the expression of complex coadapted genotypes,epistatic interactions arise between interspecific genomes in hybrids,causing postzygotic isolation (Merilä and Sheldon 1999; Orr 2001).Thus, caution has to be taken in the study of hybrids to determinegenetic architecture of species differences. It is essential to discriminateQTL that reflect the genetic structure of a trait within species from QTLresponsible for interactions in hybrids.

The parasitoid wasp Nasonia constitutes an amenable systemfor the study of behavioural differences. The three sibling species ofNasonia, N. vitripennis, N. longicornis and N. giraulti show variationsto a common pattern of male courtship behaviour (van den Assem1986; van den Assem and Werren 1994; chapters 1 and 2). Prior tobeginning the courtship a male chases a female and eventuallymounts on top of her, adopting a stereotyped position with hisforelegs on the female’s head and making contact with his mouthpartswith her antennae. The courtship is composed of consecutive cyclesof male head movements (headnods) punctuated by pauses. The first

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The role of male courtship behaviour in prezygotic isolation in Nasonia...


headnod of a series is accompanied by the release of pheromones(van den Assem and Jachmann 1980). There are species-specificvariations in the lapse of time until the male mounts on the female(“latency”), in the time it takes a male to start courting once he is ontop of the female (“fix-1st nod”), the duration of the cycles, thenumber of headnods in each cycle and the total number of cycles amale will perform before giving up if the female does not becomereceptive. One of the species, N. longicornis, performs two traitsabsent in the other two: “feet rubbing” and “minus nods”. “Feetrubbing” is the rubbing movement of the male forelegs on the femalehead, while “minus nods” are less accentuated nods between cyclesnot accompanied by the release of pheromones. Hybrid crosses areincompatible in natural populations of Nasonia due to Wolbachia-induced cytoplasmic incompatibility. In the lab, however, wasps canbe cured from bacteria with antibiotics rendering hybrid crossescompatible, despite some postzygotic incompatibility. Nasoniahybrids constitute a valuable tool for the study of the geneticarchitecture of phenotypic differences (Gadau et al. 2002).

Previous behavioural studies have found indications of non-additive effects affecting male behaviour in inter- and intraspecificcrosses (Beukeboom and van den Assem 2001, 2002; chapter 3), aswell as few factors responsible for differences in behaviour withinand between species (chapter 2). In addition, two independent QTLanalyses of hybrid males between N. longicornis males and N.vitripennis females were performed (Pietsch et al. in prep.). In thisstudy several independent coding loci were found for the differentcomponents of male behaviour on all five chromosomes, as well assignificant interactions. In our current study we performed twoindependent QTL analyses of hybrid male behaviour in the crossbetween N. vitripennis males and N. longicornis females and acommon epistatic analysis, to complement the previous analyses ofthe reciprocal cross. We compare our data to Pietsch et al. in order toidentify shared and non-shared QTL in the two reciprocal hybridpopulations.

6. QTL analysis of Nasonia ...

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Material & Methods

LinesNasonia are easily cultured in the laboratory on blowfly pupae as host.We used the same lines as Pietsch et al. (in prep.) to perform the crossbetween N. vitripennis males (line AsymCHS) and N. longicornisfemales (line IV7R2). These males are referred to as VL hybrids (Vgrandfather, L grandmother) and are the reciprocal of those used byPeitsch et al (in prep.). These lines are Wolbachia-free allowingcompatibility of hybrid crosses. Virgin hybrid females were collected byopening fly pupae before wasps emerged. Hosting of virgin femalesproduced all-male F2 offspring due to their haplodiploid sexdetermination.

Observation procedures and phenotypic traitsWe observed the behaviour of a total of 113 males that belonged totwo independent groups of hybrids. We scored “latency”, time untilmounting on the female; “fix-1st nod”, lapse of time since mountinguntil beginning of the courtship; the number of headnods in the firstfour series; duration of the first two cycles and total number of seriesuntil dismounting. We further calculated the difference between theheadnod numbers of the first two cycles (“2 minus 1”) and theprincipal component of headnods to reduce the number of headnodsin the first four series to a single variable. The “2 minus 1” and the PCof headnods are species-specific characteristics of male behaviour.The PC of headnod numbers is justified since headnod numbers in thefirst four series are highly correlated (chapter 5). Despite the highcorrelation between the duration of the first two cycles we did notcalculate the principal component because fewer than half of ourmales were scored for the second cycle duration and the data setwould have been strongly reduced. We also scored presence orabsence of “feet rubbing” and “minus nods”.

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Behavioural scoring was performed with unreceptive(previously mated) females to allow extended courtships. Males thatdid not attempt courtship after 10 minutes were discarded. We used astereo binocular microscope at 10x magnification for all observations.Individual males were placed in 60 mm glass tubes, diameter 10mm,closed off with a plug of cotton wool, and females were thenintroduced. All males were inexperienced and one day old, sincemale age and previous experience may have an effect on thecourtship performance (Beukeboom and van den Assem 2001).

Markers and linkage analysesWe performed two genotypic analyses, one with 97 and another with84 males of which 56 respectively 57 had been observed for malebehaviour. DNA was extracted from the entire wasp following astandard salt-protocol. In our first QTL analysis we used fifteenmicrosatellite markers and six AFLP (amplified fragment lengthpolymorphism) primer pairs (Appendix) that yielded a total of 200marker loci that could reliably be scored. In our second QTL analysiswe used twenty microsatellite markers and seven AFLP primer pairs(Appendix 1) that yielded 160 marker loci. The primers were used toamplify discriminating fragments of the DNA that are diagnostic for N.vitripennis or N. longicornis. Linkage groups were formed inJoinMap® (v3.0; DLO Center for Plant Breeding and ReproductionResearch, Wageningen, The Netherlands) and order of the markerswithin a group was resolved with MultiPoint®. Anchoredmicrosatellites (Rütten et al. 2004) were used to assign linkage groupsto chromosomes. In the second population chromosome IV was onlydetermined by MultiPoint® as it could not be resolved in JoinMap®.

QTL analysisUsing MapQTL® software (v5.0; DLO Center for Plant Breeding andReproduction Research, Wageningen, The Netherlands) we firstscreened the genome for QTL linked to courtship components usingthe interval mapping procedure (Lander and Botstein 1989).


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Significance of detected QTL across the genome was calculated withthe permutation test implemented in the software. We then run multipleQTL mapping (MQM) using significant peaks from interval mapping ascofactors. Cofactors absorb the phenotypic variance they account for,allowing detection of further QTL (Pietsch et al. in prep.). In caseswhere we did not find significant QTL for a trait in interval mapping atp<0.05, we selected as cofactors the peaks nearest to significance.

Epistatic analysisTo detect two-way epistatic interactions between loci we pooled thegenotypic and phenotypic data of both populations. We then used thesoftware package Epistat (Chase et al. 1997) to detect interactionsbetween primary QTL and conditional QTL. A conditional QTL haseffect only if it interacts with a primary one. To increase the chance offinding significant interactions the program was set to findinteractions with an additive likelihood of odds (LOD) of 3.0,likelihood log ratio (LLR) of 3.0 and a minimum group size of 10 (seeEpistat Manual in http://64.226.94.9). Significance of the interactions wascalculated with Monte Carlo simulations implemented in Epistat andcorrected for search over five chromosomes with Bonferronicorrections (significant p-value<0.01).

ResultsPhenotypic analysisWe scored a total of 181 F2 males, of which 113 (~60%; table 1)adopted the stereotypic position and courted the females. This rate ofcourting hybrid males is twice as high as that of the reciprocal cross,probably due to the increase in latency threshold for non courtingmales (rejected after 5 min, Pietsch et al. in prep., and after 10 min. inour present study) but may also be due to higher fitness of survivinghybrid VL males compared to LV males.

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Table 1. Phenotypic analysis of N. vitripennis (V) and N. longicornis (L) malesand hybrid males (VL) from the cross of N. vitripennis males and N.longicornis females. Sample size (N), median, quartiles, minimumand maximum values are shown. 1st - 2nd cycle: duration of the firsttwo cycles; 1st - 4th headnod: headnod numbers in the consecutivecycles; 2 minus 1: difference in headnod numbers between thesecond and the first cycles; PC headnods: principal component ofheadnod numbers; total series: number of cycles (series) performedbefore dismounting. Durational components are given in seconds.


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For almost all analysed traits except latency and number ofheadnods of the first two series, the phenotype of hybrid VL maleswas significantly biased towards N. longicornis (grandmaternalspecies). Average latency was longer than any of the pure species(Kruskal-Wallis, H=105; n=102; p<0.0001) and the number ofheadnods of the first two series was intermediate to bothgrandparental species. Longer latencies than the pure species(transgressive phenotypes) are probably an indication of low fitnessof hybrids, that are slower and weaker than pure species.Transgressive phenotypes were found for all durational components(“latency”, “fix-1st nod” and duration of the cycles; table 1) and in thetotal number of cycles performed until giving up. Hybrids did notdiffer in the total number of series, but the parental species did notdiffer either (Kruskal-Wallis, H=6; n=179; p=0.06). The two traits onlypresent in N. longicornis, “feet rubbing” and “minus nods”, werepresent in over 40% of the males, while 27% showed none of the traitsand the remainder either one or the other. This means that over 70%of the males showed at least one of the traits characteristic of N.longicornis. These results are in contrast to a previously found bias ofVL hybrids’ behaviour towards N. vitripennis (Beukeboom and vanden Assem 2001) and are consistent with nuclear-cytoplasmicincompatibility that favours recovery of N. longicornis nuclear allelesin a N. longicornis cytoplasmic background.

Linkage mapsIn the first population, 47 out of 200 markers mapped to five linkagegroups (Figure 1). In the second population, 39 out of 160 mapped tofour linkage groups (Figure 2). Linkage groups could be assigned tochromosomes by comparison to the maps in Pietsch et al. (in prep.)and Rütten et al. (2004). In the second population one linkage groupcontained on one end a microsatellite previously assigned tochromosome 2 (Nv23) and at the other end one previously assignedto chromosome 5 (Nv46). Mean recovery of the markers was notsignificantly shifted towards N. longicornis (48% of the markers were

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from N. vitripennis origin; 95% confidence intervals: 47%-50%), so wefound no indication of biased transmission ratio due to nucleo-cytoplasmic incompatibility as in other hybrid combinations ofNasonia (Gadau et al. 1999; Pietsch et al. in prep.). This is in contrastwith the observed phenotypic bias of hybrids towards N. longicornis.Thus, we find a biased expression of N. longicornis traits in a N.longicornis cytoplasmic background. Of the shared alleles (fromthree AFLP primers which yielded 41 shared fragments and 15microsatellites), only six mapped in both populations (Nv46, Nv22,Nv41, BC144, BC216 and CE143; figs. 1 and 2), and one did so in twodifferent linkage groups (BC144 on chromosome 5 in the firstpopulation and chromosome 3 in the second). This is likely due tolimited resolution as a result of the small number of individuals usedfor mapping.

QTL analysesIn the first population we found a total of 13 QTL for five out of sixanalysed traits (one for “latency”, three for “fix-1st nod”, one for theduration of the first cycle and three for the duration of the second cycle,three for “2 minus 1”, two for the PC of headnod numbers and none fornumber of series; table 2). We did not analyse the number of headnodsin the separate series because variance in the numbers is representedby the PC of headnods and the structure of the courtship (difference inheadnod numbers between the first two cycles) in the “2 minus 1”. Tenout of 13 QTL were already significant with interval mapping and theother three were found with MQM mapping. In the second populationwe found no significant QTL with interval mapping, so we chose thehighest peaks as cofactors for MQM mapping. With MQM mapping wefound a total of nine QTL for five traits (table 2). Duration of the secondcycle was not analysed in this population. The QTL explained from 29%(duration of first cycle; table 3) to 95% (“fix-1st nod”; table 2) of thephenotypic variance of the different traits. The trait “2 minus 1” thatcould not be previously mapped in Pietsch et al. (in prep.) mapped onchromosome 1 in population 2 and chromosome 1 and 5 in population


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1 It had 91% of the phenotypic variance explained in the firstpopulation, but 12% in the second. We found two QTL that explainedtransgressive phenotypes in latency and duration of the 2nd cycle inthe same region of chromosome 1. For all the traits except “latency”we found QTL of reversed effect (phenotype characteristic of onespecies with alleles of the other species) in addition to QTL with directeffect (phenotype and genotype of the same species). The region onchromosome 1 where we found transgressive QTL for latency andcycle duration also contained a QTL for “fix-1st nod” with reversedeffects. This points to a chromosomal region containing multiplegenes for courtship, or a single gene with pleiotropic effects.

We compared our QTL map to that of LV hybrids in Pietsch etal. (in prep.) (table 4). We confirmed the presence of QTL for latencyand duration for the first cycle on chromosome 1 (latency in LV andin our first VL population; duration of the first cycle in all threemapping populations); duration of the second cycle in chromosomes1 and 2 (LV and our first VL population; it was not scored in thesecond VL population); as well as QTL for headnods in chromosomesII (LV and our second VL population) and V (LV and our second VLpopulation). Four alleles linked to a QTL were shared between the LVand the second VL population, AD 309, Nv 40, Nv 37 and Nv 46, andthe latter also mapped in the first VL population. Nv 40 was linked tothe duration of the first cycle in both LV and the second VLpopulation, while AD 309 was linked to QTL for latency in LV hybridsand “2 minus 1” in our second VL population. Nv 37 and Nv 46 werelinked to duration of the second cycle respectively headnods in LV,but they did not link to any QTL in VL hybrids. Thus, although wefound some equivalence between the QTL maps, we also found somediscrepancies. Discrepancies might be due to the cytoplasmicbackground, which is V in LV hybrids and L in VL, where differentnucleo-cytoplasmic interactions might have taken place. Anothersource of disparity is the variation in resolution obtained with theparticular allelic composition of each mapping population.

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EpistasisBoth populations were pooled for the analysis of epistaticinteractions. We found significant two-way epistatic interactions induration of the first cycle, “2 minus1” and PC of headnods (table 5;figs.1 and 2). In all three occasions the interaction resulted in atransgressive phenotype of the hybrid haplotype. In the duration ofthe first cycle the N. longicornis allele at the interacting QTL wassufficient to produce the N. longicornis phenotype. In the “2 minus 1”the conditional QTL was not found with interval or MQM mapping,although it was linked to the aforementioned “CG302” marker with aseemingly pleiotropic effect on several courtship components. Theeffect of the conditional QTL on “2 minus 1” was reversed. Theconditional marker in the PC of headnods was not mapped, but it wasfound significant in interval mapping when all the genotypes andphenotypes were pooled (LOD 2.31; p<0.05). For the epistaticinteraction in “2 minus 1” and the PC of headnods, hybrids with thevitripennis allele at the conditional QTL and the longicornis allele atthe interacting one showed transgressive phenotypes, while thereversed allelic combination showed intermediate phenotype to bothpure species. In the epistatic interaction in duration of the first cycle alongicornis allele in the interacting QTL was sufficient to find thelongicornis phenotype, while the hybrid combination of longicornisallele in the conditional QTL and vitripennis in the interacting oneshowed transgressive phenotype. None of the epistatic interactionswe found in the VL hybrids correspond to the ones reported byPietsch et al. (in prep.) in LV hybrids. Since epistatic interactions inboth reciprocal crosses generally corresponded to transgressivephenotypes in hybrids, the most likely explanation is that epistasiswas dependent on the hybrid gene combination in a particularcytoplasmic background (V or L).


121


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123

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Table 4.Comparison of QTL position between LV hybrids (from Pietsch etal. in prep.) and VL hybrids. Duration of the 2nd cycle was notscored in the 2nd VL population; no QTL were found for total seriesin the first VL population and for “2 minus 1” in the LV population.

Table 5. Significant epistatic effects in the pooled VL data. Bold are reversedphenotypes; gray are transgressive phenotypes. Significance is

adjusted for search on five chromosomes.

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Discussion

We used two independent populations of hybrid males that resultedfrom crossing N. vitripennis males and N. longicornis females toidentify underlying QTL and epistatic interactions of male courtshipbehaviour. The phenotype of hybrid males was biased towards that ofN. longicornis in all courtship components except latency andheadnod numbers. Latency of hybrid males was generally longer thanpure species males, probably as a consequence of hybrid breakdownthat made hybrids slower. We found QTL for all seven components ofmale courtship behaviour on all five chromosomes explaining 29-95%of the phenotypic variance. We found epistatic interactions betweenloci responsible for transgressive phenotypes in three of the courtshipcomponents, duration of the first cycle, difference in headnodnumbers between the first two cycles (“2 minus 1”) and PC ofheadnod numbers.

Phenotypic biasPrevious studies of male courtship behaviour in hybrids between N.longicornis and N. vitripennis have shown a phenotypic bias ofheadnod numbers and cycle durations of the hybrids towards thegrandpaternal species (Beukeboom and van den Assem 2001, 2002).In our present study, phenotype of hybrid males was biased towardsthe grandmaternal species, N. longicornis, in most of the courtshipcomponents and this did not correspond to a biased recovery oflongicornis alleles. Headnod numbers were truly intermediate to bothgrandparental species, while for cycle durations our results wereconsistent with grandmaternal effects resulting from the interaction ofnuclear genes and cytoplasm in courtship behaviour. Non-additiveeffects of different nature (grandmaternal or grandpaternal effects orepistatic interactions, chapters 3 and 5) seem broadly present inNasonia. Interestingly, these effects do not appear repeatable in VLand LV hybrids. While in previous behavioural experiments headnod


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numbers and cycle durations were biased towards the grandpaternalspecies (Beukeboom and van den Assem 2001, 2002) in the currentphenotypic analysis and that of Pietsch et al. (in press) cycle durationswere biased towards the grandmaternal species and only headnodnumbers in LV hybrids appear consistently biased towards thegrandfather’s species behaviour. This lack of consistency may beexplained by interactions of genes and environment. However,repeatability of behaviour of a male on consecutive days is high(chapter 3). A possible explanation is that male behaviour isdetermined during development and depends on the alleliccomposition in interaction with the developmental environment.Thus, individual adult males have repeatable behaviours butbehavioural phenotypes are influenced by environmental conditionsduring development.

QTL and EpistasisWe found QTL with two kinds of effects in the phenotype,transgressive and within the pure species ranges. Transgressivephenotypes are probably a consequence of disrupting coadapted setsof genes in hybrids, resulting in maladaptive phenotypes (Pietsch etal. in prep.). QTL that explain phenotypes characteristic of the purespecies represent genes that have diverged either during or after thespeciation process. Among these we found QTL with direct effects(genotype and phenotype from the same species), and QTL withreversed effects (genotype from one species and phenotype from theother), with the latter often responsible for more phenotypic variancethan QTL with direct effects (tables 1 and 2). This held true even whenthe phenotypic variance was largely explained by the found QTL, asit was the case with “fix-1st nod” (94.8% of the variance explained inthe first population, with 64.3% of the variance explained by QTL withreversed effects; table2). The simplest explanation is that alleles formale courtship have not been fixed in the two species, and N.vitripennis still carries alleles responsible for N. longicornisphenotypes and vice versa. Phenotypes of the two species are

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nevertheless discrete, so alleles responsible for the other species’phenotype are not expressed. There appear to be complex interactionsfavouring expression of certain alleles in each species. In haploid malessuch interactions can only be epistatic and not due to dominance.Complex epistatic interactions would help maintaining the non-expressed alleles in the population in traits under selection (Merilä andSheldon 1999; chapter 5). Complex interactions are also consistent withthe high level of epistasis found in the analysis of the reciprocal cross(Pietsch et al. in prep.).


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AppendixComposition of AFLP primer pairs. Primer pair DD is not used in

this chapter but in chapter 7.

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Documents

University of Groningen The role of male courtship …Aitana Peire, Rense Veenstra, Christof Pietsch*, Louis van de Zande, Juergen Gadau**, Leo W. Beukeboom * Institute of Plant Genetics